Maintaining directional control while stopping a four-wheeled vehicle skidding on a slippery surface, or in a panic stop situation where the vehicle operator is applying excessive pressure to the brake pedal, requires attention to a number of interrelated problems including: getting the vehicle stopped in a reasonable distance; maintaining steering control; and controlling a condition known as yaw, where the rear wheels of the vehicle break loose before the front wheels and the rear end of the vehicle swings around toward the front of the vehicle. If yaw is not controlled, the rear end of the vehicle may swing far enough around toward the front to cause the vehicle to spin, slide sideways into an obstacle, or even overturn.
Vehicles such as light trucks, which routinely operate with only minimal weight over the rear wheels when they are not carrying a load, present special problems for brake designers in dealing with yaw. The rear brakes must be capable of stopping the truck in a required distance, as specified in government regulations, while carrying a full load and operating at the full gross vehicle weight of the truck. When the truck is operating empty, however, because there is so little weight over the rear wheels, the rear wheels tend to break loose from the driving surface under much lighter brake loads than do the rear wheels of vehicles, such as passenger cars, where the vehicle weight is more evenly distributed to the front and rear wheels.
Modern all-wheel anti-lock (AWAL) brake systems provide improved directional stability in stopping a vehicle in a skidding situation. Such AWAL brake systems typically include an electronic control unit (ECU) that receives wheel speed signals from speed sensors on the front and rear wheels. The ECU detects the impending onset of wheel lock-up by monitoring the speed of the wheels. When an impending lock-up is detected, the ECU takes control of the AWAL brake system, and rapidly pulses the brakes to prevent the brakes from locking the wheels, so that some measure of control is retained even if the braking surface is too slippery to allow normal braking.
When an impending lock-up is detected, the ECU actuates an AWAL isolation valve to close off the hydraulic connection between the brakes and the master cylinder in the base brake system, and the AWAL brake system takes over control of the brakes. A circulation pump in the AWAL brake system takes the place of the master cylinder during the operation of the AWAL brake system, and supplies a continual flow of pressurized hydraulic fluid to the brakes through apply and release valves that are opened and closed at a rapid rate by the ECU to cause the fluid pressure in the brakes to pulsate and allow a staccato rotation of the wheel. By rapidly opening and closing the apply and release valves in this manner, the brake pressure applied to the wheels can be modulated and closely controlled to maintain wheel slip within precise limits to optimize stability, steerability and stopping distance of the vehicle.
Another function typically performed by an AWAL brake system is known as dynamic rear proportioning (DRP). In performing DRP, the ECU will monitor and compare the speeds of the front and rear wheels, and control the AWAL apply and release valves in a manner that limits the pressure applied to the rear brakes to a value proportionally lower than the pressure applied to the front brakes, so that the rear brakes will always be slipping a predetermined amount in relation to the front brakes. The object of DRP is to promote enhanced directional stability through yaw reduction during braking by ensuring that the rear wheels never lock up before the front wheels. Because the AWAL brake system continually monitors the speed of the front and rear wheels during DRP operation, the AWAL system can detect differences in stopping performance of a vehicle such as a light truck when it is operating empty or carrying a load, and adjust the proportion of brake pressure applied to the front and rear wheels accordingly.
Such AWAL brake systems with DRP add considerable complexity and cost to the brake system, due to the necessity for the pump and associated controls. This additional cost and complexity has precluded installation of four-wheel ABS systems as standard equipment on all four-wheeled vehicles.
For the last several decades, however, a significant percentage of light trucks and vans that are not equipped with an AWAL brake system have been equipped with pump-less anti-lock brake systems that operate only on the rear wheels of the vehicle. In the automotive industry, such systems are sometimes known as rear wheel anti-lock (RWAL) brake systems. These systems have been shown to offer significant improvements in directional stability during braking at a lower cost than a full AWAL brake system, because RWAL systems have fewer and less costly components than AWAL brake systems.
The following U.S. patents disclose examples of prior RWAL brake systems and methods for operating such systems: U.S. Pat. No. 4,790,607 to Atkins, et al; U.S. Pat. No. 4,886,322 to Atkins; U.S. Pat. No. 4,991,103 to Lin; U.S. Pat. No. 5,487,596 to Negrin; U.S. Pat. No. 6,193,327 B1 to Atkins; U.S. Pat. No. 6,241,326 B1 to Ferguson, et al; U.S. Pat. No. 6,357,840 B1 to Atkins; and U.S. Pat. No. 6,398,321 B1 to Atkins.
As shown in FIG. 1, an RWAL brake system 10 typically includes a pedal actuated master cylinder 12 having a primary piston supplying a first volume of pressurized hydraulic brake fluid to a front brake circuit 14 connected to brakes on the left and right front wheels of the vehicle, and a secondary piston supplying second volume of pressurized hydraulic fluid to a rear brake circuit 16 to the left and right rear wheels of the vehicle. The rear brake circuit 16 includes a normally open apply valve 18, a normally closed release valve 20, a fluid storage element in the form of an accumulator 22, and a differential pressure switch 24.
The apply valve 18 has an inlet connected to the master cylinder 12 and an outlet connected to the rear brakes. The release valve 20 has an inlet connected to the rear brakes and an outlet connected to the accumulator 22. The differential pressure 24 switch is operatively connected to sense the difference between the pressure in the rear brake circuit 16 at the inlet of the apply valve 18, as supplied by the master cylinder 12, and the pressure in the rear brakes, and to generate an electrical signal when a predetermined pressure differential is detected.
The RWAL system typically includes a single wheel speed sensor 26 attached to monitor speed of a component 28, such as a rotating gear or shaft in the transmission or the differential, of the power train connecting the rear wheels of the vehicle to the engine. The single rear wheel speed sensor 26 generates a signal that is indicative of, or proportional to, an average speed of the left and right rear wheels. The RWAL system also typically includes a brake switch 30, connected to the brake pedal or the linkage leading to a vacuum booster attached to the master cylinder 12, that generates a signal indicating that the driver has depressed the brake pedal and initiated a braking event. The RWAL system may also include other sensors, such as the fluid level sensor 32 shown in FIG. 1, for performing other diagnostic and control functions.
The RWAL system further includes an electronic control unit ECU 34 connected to the apply valve 18, the release valve 20, the single wheel speed sensor 26, the brake switch 30, and the differential pressure switch 24. The ECU 34 of the RWAL system receives the signals generated by the single wheel speed sensor 26, the brake switch 30, and the differential pressure switch 24 as input signals that are processed by the ECU 34, according to analytical functions programmed into the ECU 34, to determine if RWAL operation is required during a braking event. If RWAL operation is required, the ECU 34 generates output signals for controlling the apply valve 18 and release valves 20 during RWAL operation according to control functions programmed into the ECU 34.
In general, the ECU 34 of an RWAL system controls the apply and release valves 18, 20 during RWAL operation according to various hold and release sequences that allow the ECU 34 to detect when the rear wheels are experiencing a lock-up condition, and to control the rear brakes during RWAL operation.
To reduce pressure in the rear brakes during RWAL operation, the apply valve 18 is closed to isolate the master cylinder 12 from the rear brakes, and the release valve 20 is opened to allow a portion of the pressurized fluid originally supplied to the rear brakes by the master cylinder 12 to bleed off through release valve 20. The fluid released from the rear brakes through the release valve 20 is stored in the accumulator 22, and is returned to the master cylinder following the braking event, when the driver's foot is removed form the brake pedal, through a pair of check valves 36,38 connected to allow flow from the outlet to the inlet of the apply valve 18 and release valve 20 respectively. In actual operation, the apply and release valves 18, 20 are not simply opened or closed once in a braking event, but are rather pulsed open and closed at a rapid rate by the ECU 34 during RWAL operation.
Because there is no circulation pump in an RWAL brake system for providing a continuous flow of pressurized hydraulic brake fluid, as there would be with a full AWAL brake system, the maximum volume of pressurized fluid available to the rear brake circuit 16 of the RWAL brake system 10 is limited to the volume of fluid that is supplied to the rear brake circuit 16 by a single apply stroke of the master cylinder 12. As a result, RWAL operation will terminate when the accumulator 22 is full, or when a maximum allowable portion of the volume of fluid supplied by the master cylinder 12 has been bled into the accumulator 22. The ECU 34 will terminate RWAL operation and allow the rear wheels to lock up, or apply braking force with whatever residual pressure is available to the rear brakes from the master cylinder 12, when the signals received from the differential pressure switch 24 indicate that the accumulator 22 is full.
While it would seem logical at first consideration that one could indefinitely continue RWAL operation by providing an accumulator 22 capable of storing a large volume of fluid, this is not the case, due to the limited volume of pressurized fluid available from the master cylinder 12 for use during any given braking event. Furthermore, where the front brakes are not capable, without some contribution from the rear brakes, of providing sufficient braking force to meet government standards for minimum stopping distance, the maximum allowable volume of fluid that can be bled off into the accumulator during RWAL operation will be less than the total volume of pressurized fluid supplied by the master cylinder 12 during the braking event. Where the government standards for minimum stopping distance cannot be met without the rear brakes, RWAL operation must be discontinued when the remaining pressure in the rear brakes has dropped to a minimum value required to provide the braking force needed to augment the braking force generated by the front brakes.
It will be clear to those skilled in the art that, because there is no pump in an RWAL brake system for continually supplying and re-circulating pressurized fluid, as there would be in an AWAL brake system, one must adopt a different mindset when designing an RWAL brake system, and be very stingy about how the limited volume of pressurized fluid available from the master cylinder 12 is utilized during RWAL operation.
Unfortunately, prior RWAL brake systems having only a single rear wheel speed input must utilize a portion of this limited volume of pressurized fluid for periodically performing hold and release cycles to gather data needed for determining if the rear wheels are truly experiencing a lock-up condition, and for determining when the RWAL cycle should be terminated during a given braking cycle, such as, for example, because the rear wheels having passed over a slippery patch of road surface that caused initiation of the RWAL cycle, or because the wheels have slowed enough to re-engage whatever surface they may be traversing, or that braking pressure from the master cylinder 12 has been reduced to a level that lock-up will not occur, or that the RWAL cycle must be aborted during the remainder of the braking cycle because the limited volume of fluid available from the master cylinder 12 has been transferred to the accumulator 22. The pressurized fluid wasted in performing these functions reduces the length of time that the brake system can operate in RWAL mode, and limits the effectiveness of the RWAL system during a given braking event.
In addition, the re-apply time between subsequent RWAL brake operations is affected by factors such as the temperature and volume of the fluid in the accumulator 22, and residual pressure in the rear brakes, that must bleed back to the master cylinder 12 through the rear brake circuit 16, between subsequent braking events. Periodically performing hold and release cycling of the rear brakes to determine if RWAL operation is needed, and for control during RWAL operation can undesirably lengthen the re-apply time.
It is also not possible in prior RWAL brake systems 10 to perform true dynamic rear proportioning (DRP). Because prior RWAL brake systems 10 do not utilize front wheel speed, all information relating to vehicle speed must be deduced from instantaneous rear wheel speed signals provided by the rear wheel sensor 26.
Some prior RWAL systems have included a conventional hydraulic proportioning valve 17, of the type used for several decades in vehicles with standard brake systems, and in some vehicles with various types of controlled braking systems, to provide brake proportioning at a fixed rate. These systems do not provide DRP, because the brake proportioning is not dynamically controlled by the RWAL.
As shown in FIG. 1, for RWAL systems using such conventional hydraulic proportioning valves, the valve 17 is located in the rear brake circuit. The hydraulic proportioning valve 17 has an inlet 17a connected to the master cylinder 12, to receive pressurized fluid therefrom, and an outlet 17b connected to deliver the pressurized fluid from the master cylinder 12 to the rear brakes, through the apply valve 18.
A typical pressure profile curve for a conventional hydraulic proportioning valve 17 is shown in FIG. 2, with the horizontal axis representing the front brake pressure and the vertical axis representing rear brake pressure. The rear pressure tracks and is equal to the front pressure in the region labeled 5 until a “knee” “A” is reached. Beyond the knee “A” the increase in pressure applied to the rear brakes is limited to a fixed proportion of the pressure increase applied to the front brakes, as shown by the region labeled 7, by a spring-biased mechanism in the hydraulic proportioning valve 17, in a manner well known in the art.
U.S. Pat. No. 6,241,326 B1, to Ferguson, et al, discloses a system and method for electronically controlling an RWAL system in a manner that emulates the performance of a conventional fixed-rate hydraulic proportioning valve, using a process that Ferguson calls electronic brake proportioning (EBP), in an RWAL system that does not include a hydraulic proportioning valve. When operating in the EBP mode, Ferguson uses the RWAL ECU 34 to control the isolation valve 18 in a manner that provides a pressure profile curve that approximates the curve shown in FIG. 2.
Ferguson uses signals from a rear wheel speed sensor, and/or a g-sensor, to identify when the vehicle has reached the same predetermined static deceleration threshold, i.e. the knee of the pressure profile curve, (see, for example, point A at 0.55 g on FIGS. 3 and 4) that would be utilized to initiate conventional brake proportioning, if the RWAL brake system on the vehicle included a conventional hydraulic proportioning valve. As shown by the dashed 45° line 106, in FIGS. 2 and 3, at g-levels below the predetermined static brake threshold of 0.55 g at point A, the EBP function of Ferguson applies brake pressure equally to the front and rear brakes. Once the predetermined deceleration threshold, A at 0.55 g, is reached with the RWAL operating in the EBP mode, a subroutine in the RWAL control algorithm of Ferguson controls an isolation valve 18 to hold the pressure in the rear brake circuit at a constant value as the front brake pressure continues to increase. The EBP system periodically opens the isolation valve 18, according to a pre-programmed schedule, to distribute further brake pressure increases in open-loop stepwise fashion in increments of 0.1 g as the vehicle reaches higher deceleration thresholds (see points B at 0.65 g and C at 0.75 g) to approximate an idealized proportioning curve, as shown by curves 108 and 110 in FIGS. 2 and 3 respectively that emulates a pressure profile curve for a conventional hydraulic proportioning valve.
The EBP function of Ferguson does not provide ‘dynamic’ rear proportioning, but rather uses a pre-set routine of open and hold cycles of the isolation valve that may be wasteful of the limited volume of hydraulic fluid available from the master cylinder 12, and may also result in either under or over braking of the rear wheels under conditions where the pre-set routine of open and hold cycles is not optimal for the current operating conditions being experienced by the vehicle.
What is needed, therefore, is an improved apparatus and method for providing and operating a pump-less rear wheel anti-lock brake system, that addresses one or more of the problems described above.